1. Field of the Invention
The present invention relates to driver circuits for switching solid state devices, and more particularly to driver circuits for switching silicon controlled rectifiers.
2. Description of the Related Art
Electric motors often use xe2x80x9csilicon controlled rectifiersxe2x80x9d (xe2x80x9cSCRsxe2x80x9d), which are also known as xe2x80x9cthyristors,xe2x80x9d as part of the motor""s control circuitry. An SCR can be thought of as switchable diode with three terminals: a gate, an anode, and a cathode. If a supply voltage that is less than a breakover voltage is applied across the anode and cathode of the SCR, and no xe2x80x9ctriggerxe2x80x9d current or voltage (trigger signal) is applied to the gate, the SCR is xe2x80x9coff,xe2x80x9d i.e., no current flows from the anode to the cathode. If a trigger signal is applied to the gate, the breakover voltage of the SCR drops to less than the voltage applied across the anode and cathode, and the SCR turns xe2x80x9con,xe2x80x9d i.e. current flows through the SCR from the anode to the cathode. Once on, the thyristor can remain on, provided the current through the SCR remains above a holding current, regardless of the trigger signal at the gate. To minimize the anode to cathode voltage drop across the SCR, and to minimize the power dissipated in the SCR, the trigger signal at the gate should be applied as long as current flows from anode to cathode. For the SCR to turn off, the anode to cathode current must be reduced to a level below the holding current value for the device.
A trigger circuit, or drive circuit, supplies the trigger signal to turn on the SCR. There are two variables that a designer takes into account when designing a trigger circuit. First, the power source to the trigger circuit is usually a DC voltage source that likely exhibits unwanted voltage fluctuations. These unwanted voltage fluctuations are usually caused by obtaining the DC voltage source from a step-down transformer including a full wave bridge and a capacitive input filter that are fed from an alternating current (AC) power line. The unwanted voltage fluctuations are caused by the AC power line variations and the transformer load regulation effect. Second, the gate trigger voltage, i.e., the gate to cathode voltage required to trigger the SCR varies from SCR to SCR. In addition, the necessary trigger voltage signal depends upon whether the SCR is on and conducting, or whether it is off and is to be turned on. Thus, the driver circuit must be designed to turn-on all SCRs it may encounter, i.e., all SCRs within a particular class of SCRs. Further, in order to save power, the trigger circuit must use a minimum current from the source voltage, but must still supply a sufficient voltage to trigger the SCR.
One solution to this problem has been to provide a trigger circuit that uses a switchable current source including a three terminal linear voltage regulator to provide a trigger signal sufficient to turn on all SCRs in a known class of SCRs. These trigger circuits, however, dissipate high levels of power themselves, which requires heat sinks and large supply transformers. Regulators, heat sinks, and transformers increase the size and cost of the driver circuit. Thus, there is a need for a trigger circuit to supply the appropriate trigger signal to turn on an SCR without the need for a three-terminal voltage regulator with its associated size and cost constraints.
The present invention constitutes an improvement over prior trigger circuits because it provides the required level of triggering current to turn on the SCR without a three terminal voltage regulator, thus reducing power consumption.
Systems consistent with this invention comprise a trigger circuit for triggering a silicon device having a control terminal, where the silicon device is subject to variations in the intrinsic control requirements. The trigger circuit comprises a source of direct current (DC) supply voltage, and a DC-to-DC current mode Buck converter for converting the supply voltage into an output DC current not subject to undesired variations due to variations in the supply voltage, the Buck converter supplying to the control terminal a minimum current to turn on the silicon device despite the variations in the intrinsic control requirements. The silicon device may comprise a silicon controlled rectifier (SCR) with a gate terminal, an anode terminal, and a cathode terminal, and wherein the control terminal is the gate terminal, and wherein the variations in the intrinsic control requirements are variations in the intrinsic gate-to-cathode control current and voltage requirements.
The summary and the following detailed description should not restrict the scope of the claimed invention. Both provide examples and explanations to enable others to practice the invention. The accompanying drawings, which form part of the detailed description, show embodiments of the invention, and together with the description, explain the principles of the invention.